In electrophotographic techniques used in MFP (Multi Function Peripheral), a fixing unit heats and presses unfixed toner to fix the unfixed toner on a sheet. In order that the fixing unit appropriately fixes toner on a sheet, it is necessary to apply an appropriate amount of heat per unit area of the sheet. A correlation between mass of the sheet and the amount of heat per unit area required to appropriately fix the toner on the sheet exists. Specially, the mass of the sheet is obtained by multiplying a density of the sheet by a volume thereof. Accordingly, generally, in order that the fixing unit appropriately fixes the toner on the sheet, an amount of heat per unit area required to appropriately fix the toner on the sheet is calculated on the basis of the correlation mentioned above.
The known method of calculating the amount of heat per unit area required to appropriately fix the toner on the sheet is as follows. Japanese Unexamined Patent Application Publication No. 2003-285484 discloses a technique capable of detecting a thickness of a recording sheet conveyed into the image forming apparatus, and controlling a fixing temperature.
However, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-285484, a density of the sheet is indirectly estimated by measuring the paper thickness of the sheet, and the density of the sheet itself is not measured. Thus, the technique does not focus on the density of the sheet itself. Hence, when a sheet has a thickness larger than a thickness of a normal sheet and has a density lower than a density of the normal sheet, the calculated amount of heat per unit area required to appropriately fix the toner on the sheet becomes larger than an amount of heat required in real situation. In contrast, when a sheet has a thickness larger than thickness of a normal sheet and has a density higher than density of the normal sheet, the calculated amount of heat per unit area required to appropriately fix the toner on the sheet becomes smaller than an amount of heat required in real situation. As just described, since the density of the sheet itself is not measured, a problem arises that it is difficult to appropriately calculate an amount of heat per unit area required to fix the toner on the sheet.